Renewables (wind, solar) are intermittent sources of power generation and therefore these systems require cost-effective energy storage and recovery systems to match demand with generation. Often, in regions characterized by abundant production of renewable power and weak long distance transmission infrastructure, renewable generation sources have to be curtailed, when production exceeds demand. Integrating batteries with renewable energy sources to store excess generated electrical energy is quite expensive. Other methods may include utilizing an energy storage infrastructure based on hot and cold storage tanks with molten salt, to store such excess electricity directly, or indirectly, using a compressor and turbine arrangement running with an appropriate working fluid in a heat pump mode. This energy storage system may be integrated with the molten salt storage system of a Concentrated Solar Power (CSP) plant.
However, if such excess electricity were to be directly used to heat the molten salt from the cold tank to the hot storage tank, this may be inefficient use of electricity and would not be economical to use for integration with large scale photovoltaic or wind plants. Further, indirectly heating of the molten salt using excess electricity through a heat pump cycle may achieve higher efficiency but may be limited to a maximum temperature dictated by the heat pump technology, well below the maximum temperature achievable by the molten salt of about 570° C. Generally, in the heat pump mode, the most promising fluid that may be used would be carbon dioxide (CO2), which can be compressed with commercially available compressor system to very high pressures, but at maximum temperature typically between 300° C. and 400° C.
In a heat pump cycle, higher temperature, until 600° C., though theoretically possible, is not generally practically implemented because this may necessitate the use of high performance metals and ultra-high precision manufacturing, which drastically increases the cost of such compressors resulting in an overly costly system. Further, using the standard components and achieving the targeted temperature of the molten salt may become imperative to balance the cost of addition with the loss of efficiency.
Accordingly, there is requirement of improved electrical energy storage and discharge system that uses thermal fluids to store energy and where such balance may be achieved.